1. Field of the Invention
The present invention relates to an FM receiver for receiving a frequency-modulated signal, and particularly relates to removing adjacent-channel interference and reducing audio distortion when receiving an FM transmission or the like.
2. Description of the Related Art
The frequency band necessary to transmit an FM signal is made wider than, e.g., an AM signal, in order to vary the carrier frequency on the basis of an audio signal or the like. Accordingly, when receiving a target transmission signal, an FM receiver tends to receive interference from other signals transmitted at a frequency that is close to the frequency of the target transmission signal (adjacent-channel interference), and the quality of the detected audio signal can be adversely affected by this interference. Reducing the bandwidth of a band-pass filter for extracting the target reception signal makes it possible to reduce the adjacent-channel interference. However, when the FM signal to be received is highly modulated, limiting the bandwidth may create distortion in audio signals in the detected output.
FIG. 1 is a block diagram showing the configuration of a conventional FM receiver. An RF (radio frequency) signal received by an antenna 2 is mixed with a first local oscillation signal in a first mixer circuit 4. The target reception signal is subjected to frequency conversion in order to obtain a first intermediate signal SIF1 having a predetermined intermediate frequency fIF1.
SIF1 is mixed with a second local oscillation signal in a second mixer circuit 6, and subjected to frequency conversion to obtain a second intermediate signal SIF2 having a predetermined intermediate frequency fIF2. SIF2 is passed through an IFBPF 8, which is a bandpass filter (BPF) having fIF2 as the central frequency. SIF2 is then subjected to FM detection using a detection circuit 10, and an extracted detection output signal SOUT is outputted to an output circuit comprising a speaker or the like.
The first intermediate signal SIF1 is used to generate a reception electric field strength signal SM-DC by using a signal meter (S meter circuit 14). In the S meter circuit 14, a measured signal generated on the basis of SIF1 is smoothed by a capacitor C01 and converted to direct current in order to generate SM-DC.
The IFBPF 8 is configured so as to be capable of controlling a bandwidth WF using a bandwidth controller 12. The bandwidth controller 12 switches the bandwidth WF on the basis of SOUT and SM-DC. FIG. 2 shows a block diagram of the configuration of the bandwidth controller 12. SOUT is inputted to a high-pass filter (HPF) 20 and a low-pass filter (LPF) 22.
The HPF 20 has a cut-off frequency of, e.g., approximately 100 kHz, and frequency components that exceed the audio band pass through the HPF 20. The high-pass signal that has passed through the filter is smoothed by a capacitor C02, and the terminal voltage of the C02 is inputted to a control circuit 24 as the output level of the HPF 20.
The LPF 22 transmits, for example, audio band signal components contained in SOUT. The output of the LPF 22 is inputted to the control circuit 24 via a switch 26. The switch 26 is controlled by SM-DC, and is selectively switched on when the reception electric field strength is in a predetermined weak electric field state.
The high-pass components in SOUT increase when adjacent-channel interference occurs. In response, when the control circuit 24 senses that the output level VHF of the HPF 20 is equal to or greater than a predetermined threshold dHF1, the transmission bandwidth WF of the IFBPF 8 is set to be narrower than a reference bandwidth. The effects of adjacent-channel interference can thereby be removed or reduced. When the transmission bandwidth WF is reduced, audio distortion may increase as described above. However, adjacent-channel interference has a greater effect on the audio quality than audio distortion in a state in which the reception electric field strength is maintained. Accordingly, it is preferable to remove adjacent-channel interference by restricting the bandwidth.
By contrast, in a weak electric field state, the amount of high-frequency components of noise contained in the detected output signal SOUT is higher than in intermediate or greater electric field states. Accordingly, the control circuit 24 mistakenly detects the noise components that pass through the HPF 20 as being caused by adjacent-channel interference, and the transmission bandwidth WF readily narrows. As a result, audio distortion tends to occur in a weak electric field state.
The sensitivity with which adjacent-channel interference is detected in weak electric field states has conventionally been reduced as a countermeasure for the problems described above. Specifically, in a weak electric field state, the switch 26 is switched on, and an audio band signal is inputted to the control circuit 24 from the LPF 22. The control circuit 24 senses a high modulation state in which audio distortion occurs when the output level VLF of the LPF 22 is equal to or greater than a predetermined threshold dLF. When a high modulation state is sensed, the control circuit 24 increases the threshold from dHF1 to dHF2 (>dHF1). This threshold is used to evaluate the magnitude of the output level of the HPF 20. Altering the threshold reduces the sensitivity with which the adjacent-channel interference is detected, facilitates setting the IFBPF 8 to the reference bandwidth, and lessens the likelihood of audio distortion occurring during high modulation in a weak electric field state.
When modifying the threshold described above, a problem is presented in that it is not always possible to accurately differentiate between a case in which adjacent-channel interference is actually generated and a case in which noise is present. Specifically, cases in which the threshold dHF2 is exceeded include those caused by noise, and the noise can be mistakenly detected as adjacent-channel interference, causing the bandwidth of the IFBPF 8 to be unnecessarily reduced and audio distortion to occur. Cases actually caused by adjacent-channel interference may also be included in the cases between dHF1 and dHF2. In this case, the bandwidth of the IFBPF 8 is set wide. As a result, the adjacent-channel interference is not removed, and the audio quality may be degraded. In the conventional structure, problems have thus been encountered in that it has not been possible to satisfactorily remove adjacent-channel interference and reduce audio distortion in a weak electric field state or during high modulation.
Japanese Laid-open Patent Application No. 2004-312077 is cited as a document relating to the conventional technique described above.